Crude oil and natural gas are typically recovered from subterranean reservoirs through the use of drilled wells and production equipment. After the wells are drilled, cased and cemented, it is often necessary to stimulate the reservoir by means of hydraulic fracturing or acidizing to achieve economical flow of gas and oil. This typically requires pumping an aqueous treatment fluid into the well at high rates, so that the fluid will build up pressure and cause the formation to fracture.
In the process of pumping, substantial fluid friction pressure, or drag, is observed between the treatment fluid and the tubing or casing as the fluid reaches turbulent flow, thus causing a substantial energy loss. As a result of the energy loss, a higher pumping pressure is needed to achieve the desired flow rate and pressure. It is therefore common to include drag-reducing additives in the aqueous treatment fluids to suppress the turbulence and realize lower pumping pressures. Common drag-reducing additives include oil-external emulsions of polymers with oil-based solvents and an emulsion-stabilizing surfactant. The emulsions may include guar-based or polyacrylamide-acrylic acid (PAM-AA) copolymers. Typically these prior art emulsions consist of an aqueous phase dispersed in a non-aqueous phase, in a weight ratio of from about 5:1 to about 10:1 aqueous phase to non-aqueous phase.
The surfactants in known drag reduction emulsions are typically emulsifying surfactants that stabilize the emulsions. The emulsifying surfactants have low HLB values, generally between 4 and 8. The transfer of the polymer from inside the aqueous phase of the oil-external emulsion into an aqueous treatment fluid is achieved by the inversion of an emulsion. A common way to achieve this inversion is to use an inverting surfactant, which is typically water-soluble and has an HLB of greater than about 7. Inverting surfactants may be a part of polymer emulsion formulations or may be added to a solution into which the emulsion is to be inverted.
The problem encountered with these known treatments, however, is that inverting surfactants may adversely interact with the emulsifier or emulsion and destroy it prior to use. Thus, commercially available polymer emulsions generally contain less than 5% of inverting surfactant. Polymer emulsions with this low amount of inverting surfactant, however, may not provide the desired reduction in friction because the polymer emulsion either does not invert completely or is not brine or acid tolerant.
In the event that acid or high salt contents are encountered, emulsion copolymers of 2-Acrylamido-2-methyl propane sulfonic acid (AMPS) are commonly used. These AMPS copolymers, however, may be cost prohibitive. In either case, the high molecular weight polymers may also cause substantial damage to the formation permeability. Thus, there is a continued need for more effective compounds that are more efficient, more salt tolerant, and less damaging.